1. Field of the Invention
The present invention relates to a head driving device, a recording head unit, and an image forming apparatus.
2. Description of the Related Art
As an image forming apparatus, such as a printer, a facsimile machine, a copier, or a digital printer, there is a known image forming apparatus of a liquid ejection recording system (for example, an inkjet recording apparatus) that uses a recording head including a liquid ejection head (droplet ejection head) for ejecting ink droplets, for example. The image forming apparatus of the liquid ejection recording system ejects ink droplets from the recording head onto a recording medium (for example, a sheet of paper) to form a desired image.
The recording head includes a nozzle for ejecting ink droplets, an ink channel (pressure chamber) communicating with the nozzle, and a pressure generating means that pressurizes ink in the ink channel. As the recording head, a so-called piezoelectric type, a thermal type, an electrostatic type, and the like are generally known. A piezoelectric type recording head uses a piezoelectric element (for example, a piezo element) as the pressure generating means, and causes a diaphragm forming a wall surface of the ink channel to slightly vibrate with the aid of the piezoelectric element in order to change the inner capacity of the ink channel to eject ink droplets. A thermal type recording head heats the ink in the ink channel by using a heat resistant element in order to generate air bubbles and cause pressure to occur to eject ink droplets. An electrostatic type recording head includes a diaphragm, which forms a wall surface of the ink channel and which is arranged so as to face an electrode, and deforms the diaphragm by an electrostatic force generated between the diaphragm and the electrode in order to change the inner volume of the ink channel to eject ink droplets.
In general, the recording head includes a plurality of nozzles for ejecting ink droplets, and includes an ink channel (pressure chamber) and a pressure generating means (hereinafter, an example will be described in which a piezoelectric element is used as the pressure generating means) for each of the nozzles. The nozzles are arrayed in a predetermined direction. Hereinafter, this direction is referred to as a nozzle array direction.
All of the piezoelectric elements are electrically connected in parallel between a common power supply line and a ground line, and switching elements are electrically connected in serial to the respective piezoelectric elements. Signals (drive waveforms) for driving the piezoelectric elements are generated by a drive waveform generating circuit, and selectively distributed and supplied to each of the piezoelectric elements via the power supply line and the switching element. Specifically, when a predetermined switching element is selected and turned on based on print data, a drive waveform is applied to the piezoelectric element via the power supply line, and ink droplets are ejected from a predetermined nozzle corresponding to the piezoelectric element to which the drive waveform is applied.
Further, there is a known recording head that ejects a plurality of types of ink droplets (for example, a large droplet, a medium droplet, and a small droplet) with different ink volumes in order to change the size of a dot formed on a recording medium to improve the gradation of an image. When this recording head is used, a drive waveform is set so as to sequentially eject ink droplets with changing the droplet velocity by a drive waveform of a plurality of pulse trains in a print cycle such that the droplets coalesce into a single droplet while the droplets are flying. As a system for driving the recording head as described above, a common drive circuit system is generally employed, which selectively applies a necessary waveform portion to each of the piezoelectric elements by a switching element by using a single common drive waveform that is a combination of a plurality of drive waveform components for ejecting a plurality of types of ink droplets.
Meanwhile, the drive waveform for driving the piezoelectric element normally needs to be a waveform with a relatively large voltage magnitude of 20 volts (V) to 40 V, and a drive waveform generating circuit for generating and driving the waveform is relatively large in size and power consumption. Therefore, it is often the case that the drive waveform generating circuit is not arranged inside the recording head that needs to be downsized, and a drive waveform generated by a different circuit board is supplied to the recording head through a power supply line. Further, the switching element provided for each of the piezoelectric elements is usually integrated with a control unit or the like that generates an ON/OFF selection signal, and arranged near the piezoelectric element inside the recording head. The integrated switching element is configured by a transistor, uses a high-voltage power MOSFET or the like to drive a relatively large voltage magnitude, and is large in size. Therefore, a ratio of the size of the switching element to the size of the integrated circuit is large.
To form a high-quality image in the image forming apparatus of the liquid ejection recording system, it is necessary to eject a desired amount of ink droplets onto a desired position on a recording medium. Therefore, the drive waveform supplied to the piezoelectric element is appropriately set by taking into account the ink droplet velocity, the stability of an ejection state (curved ejection, satellite, mist generation status), or the like.
However, there is more than a small amount of manufacturing variation in elements and members of each of the nozzles, such as variation in the shape of the nozzle of the recording head, the structure of the ink channel, the characteristics of the piezoelectric element, or the characteristics of the switching element. Therefore, in the conventional common drive circuit system, even when a drive waveform that is appropriately set by taking into account the ink droplet velocity or the stability of the ejection state is used, an ink droplet amount or a landing position may vary for each of the nozzles due to the above described variation, resulting in the reduced image quality.
As one solution for the above described issue, a technology as described in Japanese Patent No. 4764690 has been proposed. The technology described in Japanese Patent No. 4764690 is to select, for each of pressure generating elements, a drive waveform generating circuit for applying a drive signal waveform to each of the pressure generating elements from among a plurality of drive waveform generating circuits so as to cause liquid ejected from a plurality of nozzles to land in an approximately linear manner along the nozzle array direction.
However, the shape of the nozzle, the structure of the ink channel, the characteristics of the piezoelectric element, and the characteristics of the switching element independently vary. Therefore, to cope with all the variation by the technology described in Japanese Patent No. 4764690, a large number of drive waveform generating circuits are needed.
Further, even with a drive waveform that is appropriately set so as to sequentially eject ink droplets with changing the droplet velocity such that the droplets coalesce into a single droplet while the droplets are flying, it may be difficult to cause the flying droplets to coalesce into a single droplet because of a change in the droplet velocity due to the above described variation. In this case, it may be difficult to eject a desired amount of droplets onto a desired position, resulting in the reduced image quality. Specifically, the ejection characteristic varies for each of different types of ink droplets (a large droplet, a medium droplet, and a small droplet) with different ink volumes, so that if the variation is to be dealt with by the technology described in Japanese Patent No. 4764690, the number of combinations of appropriate drive waveforms becomes huge.
Therefore, in the technology described in Japanese Patent No. 4764690, to correct variation in the ink droplet amount and the landing position with high accuracy in order to prevent a reduction in the image quality, the size of the drive waveform generating circuit, the size of the switching element, and the size of the power supply line for supplying the drive waveform to the recording head increase. Consequently, the size of the apparatus, the power consumption, and the costs increase, which makes practical implementation difficult. In contrast, if the number of the drive waveform generating circuits is reduced within a realistic range, the correction accuracy for the variation in the ink droplet amount and the landing position is insufficient, which is inadequate to improve the image quality.